1. Field of the Invention
The present invention relates to a system and method for printing an image and transfer to media. More particularly, the present invention relates to an inkjet printer system having optimum image transfer and optical density by generating pressure uniformity across a transfer nip, controlling release coating thickness, and controlling a time delay between a jetting of ink and transfer of jetted ink to media.
2. Description of the Related Art
FIG. 1 illustrates an example of a conventional inkjet printer, including inkjet printer 10 having a platen or tray for receiving media, such as paper, a carriage 15 to carry ink cartridge(s) 17, and a processor 20 to control the operation of inkjet printer 10. As illustrated in FIG. 1, carriage 15 carries ink cartridge(s) 17 across media or potentially axially across intermediate transfer medium (ITM) 40, as further illustrated in FIG. 2. In FIG. 2, related to embodiments of the present invention, ink cartridges 27 travels across ITM 40 while printing onto ITM 40 in spiraled swaths, i.e., helical printing. Once the printing to ITM 40 has completed, media 30 is compressed between transfer nip 60 generated at the intersection of ITM 40 and transfer roller 45, thereby transferring the image printed on ITM 40 to media 30.
Although the example illustrated in FIG. 2 shows helical inkjet printing onto ITM 40, many alternative techniques are also available, such as printing in stepped swaths across ITM 40, for example.
FIG. 2 also illustrates that a release coating may be applied to the surface of ITM 40. The release coating is used to improve print quality, as well as release and transfer of an image from an ITM to media. The release coating is referred to as a release material because it acts as a release layer between the ITM and ink layer. In an embodiment of the present invention, illustrated in FIG. 3, ink is jetted onto the liquid coating by four print heads 37 placed around the circumference of ITM 40. As illustrated in FIG. 4, the ink can be jetted onto ITM 40 in helical direction 80 as print head(s) 47 travels axially across ITM 40, while ITM 40 rotates in direction 84. FIG. 4 also illustrates that that a portion of ITM 40 may not be printed upon to allow print head(s) 47 to properly advance to the next helical swath printing position, without overlap. FIG. 4 illustrates this portion of ITM 40 not printed upon between the leading edge 70 and trailing edge 75 of the ink image. This illustration in FIG. 4 also illustrates that ITM 40 can be a drum with an external circumference sufficient to transfer ink to the whole print area of the media in one revolution. As illustrated in FIG. 5, ITM 40 could transfer the whole printed image to media traveling through the transfer nip in one revolution.
As illustrated in FIG. 2, transfer roller 45 can be moved into contact with ITM 40 to form transfer nip 60. Transfer roller 45 may be moved into contact with ITM 40 while ITM 40 is stopped, or transfer roller 45 may be moved into contact with ITM 40 while it is rotating. Media 30 is fed into transfer nip 60, and the pressure between ITM 40 and transfer roller 45, at transfer nip 60, enables the transfer of the jetted ink from ITM 40 to media 30. If the ink transfer isn't optimized, ink will remain on ITM 40, which would require ITM 40 to be cleaned before ink can again be jetted to form the next image.
To provide an improved image quality, release coating compositions may also destabilize a colorant in ink prior to penetration into the media. The colorant in ink might be dyes, pigments, or other materials, depending on the chemical structure of the ink. Similarly, the release coating composition is designed to interact with a corresponding ink. For example, the release coating may include a flocculant, such as a liquid that contains a multivalent salt or is low pH, which may be applied to an ITM before, during, or after the jetting of ink. When the ink impacts the flocculent, the colorant in the ink destabilizes, thereby preventing penetration of the colorant into media while allowing penetration of the remaining ink constituents. Further, in this example, a mordant may also be added to the liquid composition to reduce spreading and color-to-color bleeding of the ink.
If the release coating is a low viscosity liquid, then foam rolls or felt wicks can be used to apply coatings. Very thin fluids can also be jetted via inkjet-like print heads. If fluids are of a higher viscosity (to allow the use of additives which give improved print quality or provide more rapid ink absorption effects), then more complicated application methods such as blade coating or roll coating become necessary. Both traveling coaters and page-wide coaters would be available. Apparatuses and methods for applying liquid release coatings are known, and for brevity not discussed further herein.
Examples of ITM printing systems using release coatings are described in U.S. Pat. Nos. 5,389,958, 5,805,191, and 5,677,719, all of which describe release coating material on an ITM, jetting ink onto the coated surface of the ITM, and thereafter transferring the ink image to media through a nip generated by the ITM and a roller. Liquid coating systems require fluid handling hardware, including subsystems to store fluids, to move them from the storage vessel to the coating system, to apply them to an ITM, and to clean off residue after image transfer. Examples of such liquid coating techniques have also been illustrated in U.S. Pat. Nos. 6,183,079 and 6,196,674.
In FIG. 2, the ink image on ITM 40 is transferred to media 30 by pressure at transfer nip 60. An uneven pressure profile in the transfer nip will result in non-uniform print quality across the page. The generation of a uniform pressure profile can be addressed by making the transfer roller a solid shaft, though solid shafts tend to be massive and costly to actuate. A solid shaft, which has a large mass and rotational moment of inertia, will also cause velocity fluctuations when it comes into contact with ITM 40, since the transfer roller must typically come into contact and dampen out any velocity fluctuations within the no-print zone.
In embodiments of the present invention, an ITM was initially operated with a surface speed between 26.6 to 53.3 ips, depending on the print mode. Upon operation with a drum ITM, with a circumference of 9.5 inches, surface speed of 53.3 ips, and a one inch no-print zone, with transfer roll being engaged and stabilized in 18 ms, it was determined that a massive transfer roller was needed to prevent a non-uniform pressure profile across the width of the transfer nip, which would result in degraded ink and image transfer. To reduce the problems associated with such a massive transfer roll, a hollow roll was used. The hollow roll reduced the mass in the system but also reduced the stiffness of the transfer roller. The hollow transfer roller produced larger deflection at the center of the roller, which translated into a non-uniform pressure profile.
Further examples of generating uniform pressure profiles are described in U.S. Pat. No. 5,092,235. U.S. Pat. No. 5,092,235 fails to address the additional factors of release layer thickness and ink transfer delay time the present inventors have further discovered.
As briefly noted above, a release coating is necessary when printing using an ITM. When ink is placed between two solid bodies of similar surface energy, and the two solid bodies are pulled apart, the ink is caused to split, leaving fluid on both solid bodies. With the ink system used, anything close to a 100% transfer of ink cannot be achieved unless a sacrificial layer is placed between the ink and one of the solid bodies. In this case, the sacrificial layer is the release coat. It was determined that depending on the thickness of the release material and on the rheological properties (such as cohesive strength of the release layer), different transfer efficiencies could be achieved.
Transfer is also time dependent. If the image is left on the drum for an extended amount of time, the ink begins to dry and transfer is affected. If the image is transferred too soon the pigment does not have time to flocculate, resulting in poor print quality.
Embodiments of the present invention overcome these aforementioned problems while optimizing image release and optical density.